In particular in the case of Diesel combustion engines, it is of importance that the level of soot particulates emitted into the environment be kept as low as possible. To this end, it is useful to monitor the soot particulate emission while the combustion engine is in operation using a sensor mounted in the exhaust branch. In this context, the sensor can be placed downstream or upstream of a soot filter mounted in the exhaust branch. If the sensor is placed in the exhaust branch downstream of the soot filter, the sensor can also be used for monitoring the performance of the soot filter.
Moreover, the concentration of soot particulates in an exhaust gas of a combustion engine may be measured continuously, that is during the normal operation of the engine, to allow direct detection of changes in the engine behavior, for example, because of a failure.
A sensor of the type mentioned at the outset is referred to in German Patent Application DE 198 53 841 A1. This sensor is used in particular to detect soot particulates in an exhaust branch of a motor vehicle having a Diesel combustion engine and includes a first electrode or center electrode, which is connected to a high voltage source, as well as a second electrode or ground electrode, which is at the same potential as the exhaust branch made from metal. The measure used for the concentration of soot particulates in the exhaust gas is either the minimum voltage level at which sparking occurs between the two electrodes, or else, if the electric voltage is kept constant, the magnitude of the ionization current flowing between the two electrodes.
In the case of filters used in conjunction with Diesel engines or in other methods for particulate retention, loading or clogging of the filter can occur and be detected by differential pressure measurement, the differential pressure being measured upstream and downstream of the filter. Until now, no methods, in particular, no methods suitable for mass application in automotive applications are known to allow measurement of leaks or cracks in such a soot filter during operation. However, such leaks or cracks impair the quality of a differential pressure measurement by pressure equalization and considerably increase the emission of particulates.